Since you haven't recently changed your lifestyle dramatically, your back pain is most likely due to muscle strain, sprain or other minor injury from your daily activities. The pain should go away over time.
To decrease pain, avoid overuse, rest briefly if necessary (but avoid complete bedrest for more than a day or two as this appears to provide little benefit in most cases), and avoid those activities that might have triggered your pain in the first place. Over-the-counter pain medications, topical medications (such as Icy Hot or Ben Gay), or the application of ice may be helpful.
If symptoms are getting worse or do not go away, consult your physician.